Manufacturing and assembly of composite materials often now involves assembling structures using bonded joints, such as room temperature adhesive paste bonded joints, instead of traditional fasteners. In addition to inspecting the bondline for potential damage or flaws, such as voids or cracks, verifying bondline thickness can be crucial to ensuring proper strength of the bonded joint. For example, if the bondline is too thick, the joint can be weakened. Accordingly, it is useful to be able to verify the thickness of bondlines, including remotely located bondlines which may not be easily accessed for measurement, either continuously along the length of the bondline or across the area of the bonded joint or spot checks of thicknesses at particular locations. Conventionally, however, often the only way to measure bondline thickness is to remove a part to allow for measurement of the bondline or to drill a hole into the structure to permit insertion of a measurement tool and then repair the damage.
Non-destructive inspection (NDI) of structures involves thoroughly examining a structure without harming the structure or requiring significant disassembly of the structure. Non-destructive inspection is typically preferred to avoid the schedule, labor, and costs associated with removal of a part for inspection, as well as avoidance of the potential for damaging the structure, such as drilling a hole into a structure for insertion of a measurement tool. Non-destructive inspection is advantageous for many applications in which a thorough inspection of the exterior and/or interior of a structure is required. For example, non-destructive inspection is commonly used in the aircraft industry to inspect aircraft structures for any type of internal or external damage to or flaws in the structure. Inspection may be performed during manufacturing of a structure and/or once a structure is in-service. For example, inspection may be required to validate the integrity and fitness of a structure during manufacturing and ongoing and future use while in-service, including verifying the thickness of a bondline. However, access to interior surfaces and bondlines is often difficult or impossible without disassembly or damage to the structure, such as removing a part or drilling a hole into a structure for insertion of a measurement tool.
Among the structures that are routinely non-destructively tested are composite structures, such as composite sandwich structures and other adhesively bonded panels and assemblies, many with interior bonded joints, such as on the interior walls of an aircraft. In this regard, composite structures, such as composite stiffeners and sandwich (honeycomb) fuselages, are commonly used throughout the aircraft industry because of the engineering qualities, design flexibility and low weight of composite structures, such as the stiffness-to-weight ratio of a composite sandwich structure. As such, it is frequently desirable to inspect composite structures to identify any flaws, such as cracks, voids or porosity, which could adversely affect the performance of the composite structure and, as previously mentioned, thicknesses of bondlines which could weaken structures and the overall product. However, as mentioned above, many bonded joints are located on the interior of a product where the bondline may be inaccessible for inspection.
Various types of sensors may be used to perform non-destructive inspection. One or more sensors may move over the portion of the structure to be examined, and receive data regarding the structure. For example, a pulse-echo (PE), through transmission (TT), or shear wave sensor may be used to obtain ultrasonic data, such as for thickness gauging, detection of laminar defects and porosity, and/or crack detection in the structure. Resonance, pulse echo or mechanical impedance sensors may be used to provide indications of voids or porosity, such as in adhesive bondlines of the structure, and measurement of bondline thickness. For example, conventional measurement of bondline thickness using pulse echo ultrasonic inspection may involve an ultrasonic test system with a pulser/receiver card for sending an electronic impulse signal to a pulse echo ultrasonic transducer, which translates the electronic impulse signal into an ultrasonic pulse. The ultrasonic pulse (or stress wave) travels through the structure (or part) under inspection and is partially reflected at the near and far sides of the bondline. The reflections return to the pulse echo ultrasonic transducer, which translates the reflected pulses back into electronic signals, which are communicated back to the pulser/receiver card for further analysis by the pulser/receiver car or a processing element of the ultrasonic test system and/or presentation by the ultrasonic test system. Typically, the reflected pulses are shown on a display screen, graphically and/or in some form to provide data regarding the time differential between the reflected pulses. The difference in time between the two reflections from the near and far sides of the bondline is used to calculate the thickness of the part. Half the time difference between the near- and far-side reflections multiplied by the wave speed in the bond material is equal to the thickness of the bondline, as provided by the following equation.
                                          Δ            ⁢                                                  ⁢            t            ×            wavespeed            ⁢                                                  ⁢            in            ⁢                                                  ⁢            adhesive            ⁢                                                  ⁢            bond            ⁢                                                  ⁢            material                    2                =                  bondline          ⁢                                          ⁢          thickness                                    Eq        .                                  ⁢        1            Halving the time difference multiplied by the wave speed accounts for the stress wave twice passing through the thickness of the bond material, first when traveling through the bond material to the far side of the bondline and second when reflecting from the far side of the bondline back toward the pulse echo ultrasonic transducer. Software of an ultrasonic test system may command a processing element, such as a computer processor, to automatically calculate the bondline thickness from the time differential of the reflections and a known wave speed through the bond material. A known wave speed through the bond material may be determined, and the system calibrated, using a calibration block with various bondline thicknesses of the bond material.
Non-destructive ultrasonic testing often involves coupling an ultrasonic signal from a transducer or transducer array to the surface of the structure under inspection, such as bubbling water between an inspection device and the structure. While solid laminates may be inspected using one-sided pulse echo ultrasonic (PEU) testing and bondline thickness may be measured using one-sided pulse echo ultrasonic testing, composite sandwich structures typically require through-transmission ultrasonic (TTU) testing for high resolution inspection. In through-transmission ultrasonic inspection, ultrasonic sensors such as transducers, or a transducer and a receiver sensor, are positioned facing the other but contacting opposite sides of the structure to be inspected such as opposite surfaces of a composite material. An ultrasonic signal is transmitted by at least one of the transducers, propagated through the structure, and received by the other transducer. Data acquired by sensors, such as PEU and TTU transducers, is typically processed by a processing element, and the processed data may be presented to a user via a display.
Non-destructive inspection may be performed manually by technicians who move an appropriate sensor over the structure. Manual scanning generally consists of a trained technician holding a sensor and moving the sensor along the structure to ensure the sensor is capable of testing all desired portions of the structure. In many situations, the technician must repeatedly move the sensor side-to-side in one direction while simultaneously indexing the sensor in another direction. For a technician standing beside a structure, the technician may repeatedly move the sensor right and left, and back again, while indexing the sensor between each pass. In addition, because the sensors typically do not associate location information with the acquired data, the same technician who is manually scanning the structure must also watch the sensor display while scanning the structure to determine where the defects, if any, are located in the structure. Similarly, non-destructive bondline thickness measurement inspection often requires a technician to locate and map the position of a bondline and then inspect the bondline, such as by passing a pulse echo ultrasonic transducer device along the bondline. The quality of the inspection, therefore, depends in large part upon the technician's performance, not only regarding the motion of the sensor, but also the attentiveness of the technician in interpreting the displayed data. More particularly, a technician's ability to perform inspection often is limited by the access the technician has to a location for placing a pulse echo ultrasonic sensor, such as at a remotely located internal surface position. For example, pulse echo ultrasonic measurement of a bondline on a composite sandwich structure is only possible from the bonded side of the structure because the core, often honeycomb, structure is a barrier to pulse echo stress waves.
Accessibility to the structure and, particularly for bondlines, accessibility to a particular side of a structure for inspection are considerations in choosing a non-destructive inspection device a limitation to being able to perform certain non-destructive inspection activities, such as measuring a remote bondline thickness. For example, access to a remote bondline (a bondline located in a remote position) may be so limited or inaccessible that a manual inspection by a technician is not possible or would require damage to the structure. Alignment and positioning of sensors such as, pulse echo ultrasonic transducers and devices therefor, is similarly complicated by accessibility to the structure such as inaccessibility to one side of a composite sandwich structure.
Accordingly, a need exists for an improved non-destructive inspection device and method to inspect a structure, particularly for measurement of remote bondline thickness.